1. Field of the Invention
The present invention relates to maturated, unsaturated polyester resin molding compositions or compounds which are especially useful for making reinforced plastic articles in molding apparatus having heated surfaces utilizing such molding compositions.
2. Description of the Prior Art
Unsaturated polyester resin compositions are extensively used in modern industry for the manufacture of precisely-formed, durable articles of many types. Such compositions include an ethylenically unsaturated polyester resin which is the polyesterification reaction product of polyhydric alcohols and polycarboxylic compounds such as polycarboxylic acids or polycarboxylic acid anhydrides wherein at least a portion of the polycarboxylic compounds contain ethylenic unsaturation. The unsaturated polyester resin is combined with suitable cross-linking compounds having a terminal EQU &gt;C.dbd.CH.sub.2
radical. In order to fill the needs of high-speed efficient manufacturing processes, it is required that such compositions be rapidly cured in most applications to minimize process curing time. Rapid curing compositions are particularly desirable where the unsaturated polyester resin or resins are combined with essentially inert fillers which usually include fibrous reinforcement to form molding compounds having desired chemical and physical properties. Such molding compounds are commercially used in conjunction with matched metal dies which are frequently operated at elevated temperatures and pressures in making intricately-shaped articles.
Polymerizable polyacrylates of polyepoxides are known in the art as being capable of homopolymerization and of copolymerization with unsaturated polyesters as disclosed in U.S. Pat. Nos. 3,256,226, 3,301,743 and 3,317,465.
Unsaturated polyester resin compositions are prepared by reacting polyhydric compounds with polycarboxylic compounds under esterification conditions. The polycarboxylic compounds may be polycarboxylic acids or acid anhydrides as long as a substantial portion of the selected polycarboxylic compound contains ethylenic unsaturation.
Typical polyhydric compounds include ethylene glycol, polyethylene glycol, propylene glycol, poly propylene glycol, butylene glycol, polybutylene glycol, and the like. Typical carboxylic compounds include phthalic acid, phthalic anhydride, isophthalic acid, terephthalic acid, adipic acid, oxalic acid, endomethylene tetrahydrophthalic anhydride, endomethylene tetrahydrophthalic acid, hexachloroendomethylene tetrahydrophthalic anhydride, maleic acid, maleic anhydride, fumaric acid, itaconic acid, citraconic acid, and the like.
The resulting polyester resin is normally dissolved in a suitable unsaturated cross-linking monomer containing terminal Ch.sub.2 =CH-groups, such as styrene, vinyl toluene, divinyl benzene, methylmethacrylate, ethylmethacrylate, methylacrylate, ethylacrylate, acrylic acid, methacrylic acid, diallyl phthalate, triallyl cyanurate, orthochlorostyrene, and the like.
Usually the unsaturated polyester resin composition contains a suitable polymerization inhibitor such as hydroquinone, quinone, alkyl phenols, and the like, to prevent premature gelation. The materials are cured to a hardened thermoset condition by the addition of suitable free radical polymerization initiators such as organic peroxy compounds such as tertiary butyl perbenzoate, benzoyl peroxide, dicumyl peroxide, and the like. The catalyzed resin compositions are normally heated to obtain more rapid curing.
In the past, molding compositions have been frequently formulated containing from about 15 to 55 parts by weight of the unsaturated polyester resin composition and from about 85 to 45 parts by weight of essentially inert fillers and/or reinforcing fibers including carbonates, silicates, clays, glass fibers, mineral fibers, pigments, and the like. The molding compositions also frequently contain a small quantity of a mold release agent.
Previously, it has been found that the cure time required for making molded articles from unsaturated polyester resin molding compositions could be lowered to commercially desirable periods without adversely affecting the quality of the cured resinous products. This was achieved in molding apparatus having heated molding surfaces by incorporating into conventional molding compositions from about 0.01 to 0.20 parts by weight of a polyacrylate based on the weight of the unsaturated polyesters in the composition. The polyacrylate is the reaction product of a polyepoxide and an ethylenically-unsaturated carboxylic acid which may be methacrylic acid or acrylic acid. The polyepoxide and carboxylic acid are combined in stoichiometric proportions to provide about 1 mole of the acid for each expoxide group. The cure time for articles molded with the stated amount of polyacrylates of polyepoxides is less than about one-half that required for molding compositions containing the identical unsaturated polyester resins without the polyacrylates of polyepoxides. Such improvement is disclosed by U.S. Pat. No. 3,621,093 to Svoboda et al., entitled, "Process For Making Reinforced Thermoset Articles", issued Nov. 16, 1971.
The concept of improving the viscosity temperature dependence of maturated moldable unsaturated polyester resin systems using dual thickening agents consisting of an oxide or hydroxide of magnesium or calcium and a polyisocyanate is known in the art. Such dual thickening system provides a greatly-improved viscosity index resulting in a lesser viscosity decrease with increase in temperature as encountered in conventional heat and pressure molding of precision parts. Such improvement is disclosed by U.S. Pat. No. 4,067,845 to Epel et al., entitled, "Maturation of Polyester Compositions For Viscosity Index Control", which patent is assigned to the same common assignee as the present application.